1. Field of the Invention
This invention relates to a bandpass filter having a plurality of cascaded wave-guide cavities. In particular, this invention relates to an improvement in dual-mode filters whereby quasi-orthogonal coupling structures are introduced into the cavities.
2. Description of the Prior Art
A dual-mode cylindrical and/or cuboid longitudinal filter structure having input and output ports at opposing ends of the filter is described in U.S. Pat. No. 3,697,898 to Blachier and Champeau. The dual-mode structure allows for two independent orthogonal modes to resonate in a single cavity structure, thus reducing by half the number of physical cavities required in a filter with cavities resonating in a single mode. In addition, the coupling structures in a dual-mode filter allow for positive and/or negative feed back couplings to take place between non-sequential modes. This non-sequential coupling capability can result in an enhancement of the phase characteristics in the passband and/or an enhancement of the amplitude characteristics at the skirts of the filter response.
An improvement to the Blachier and Champeau filter is described in U.S. Pat. No. 4,060,770 to Atia and Williams. The Atia and Williams canonical filter provides the means to realize the most generalized bandpass filter response within the constraints of the synthesis method detailed in an article by Atia, Williams and Newcomb entitled, "Narrow-Band Multiple-Coupled Cavity Synthesis", published in the Institute of Electrical and Electronics Engineers Transactions on Circuits and Systems, Vol. CAS-21, No. 5, September, 1974, at pages 649 to 655. This synthesis method can only be used to design filters which are synchronously tuned and which have symmetrical amplitude and phase frequency response in the dispersionless case. Generally, the Atia and Williams filter has the input and output located in the same cavity rather than at opposite ends of the filter as described by Blachier and Champeau.
An additional filter is described in an article by Pfitzenmaier in the 1977 Institute of Electrical and Electronics Engineers MTT-S International Microwave Symposium Digest, at pages 400 to 403. This filter allows the same generalized bandpass filter response to be realized as the Atia and Williams filter. However, the form of coupling matrix is symmetrical for the Atia and Williams filter and asymmetrical for the Pfitzenmaier filter. Physically, the Atia and Williams filter has the input and output ports in the same dual-mode cavity while the Pfitzenmaier filter has the input and output ports in different dual-mode cavities.
The filters described above generate a geometrically symmetrical amplitude response and an asymmetrical phase response. The asymmetrical characteristic of the phase response is due to wave-guide dispersion and cannot be controlled or compensated by filters described above without destroying the equi-ripple character of the passband response.
It is an object of the present invention to provide a bandpass filter that can produce amplitude and phase responses that are either symmetric or asymmetric, as desired. It is a further object of the present invention to provide a bandpass filter that can be made to produce the most general class of electrical response permissible in a dual-mode structure.